This invention relates to a griddle, in general, and, more particularly, to a griddle that is particularly adapted for use in the food service industry.
The food service industry involves the preparation of substantial quantities of food for consumption by a large number of people. Thus, institutions involved in the food service industry include cafeterias, hospitals, colleges, prisons, restaurants, airlines, etc.
My prior U.S. Pat. No. 5,553,531, the disclosure of which is incorporated by reference herein, discloses a multi-functional cooking device that combines a steamer, skillet, griddle, fryer, kettle, holding cabinet and oven into a single unit. The device of this invention is a griddle which can be used in the multi-functional cooking device of my prior patent, or can be used as a free-standing griddle wherever food is prepared for consumption by the public. It can be used in any environment in the food service industry and the specific environment or device in which it is used forms no part of this invention.
In my prior patent, the griddle has a griddle plate formed from stainless steel, or in a modified embodiment, the stainless steel can be clad with another heat conductive metal, such as aluminum, copper or steel. The griddle plate used in the commercial embodiment of the invention disclosed in my prior patent consisted of a stainless steel plate to which was clad carbon steel. It is known to clad household pots and pans with copper, in order to increase heat transfer. However, it is not believed that stainless steel has ever been clad with copper to form a griddle for commercial purposes.
This invention recognizes that the combination of copper and stainless steel provides significant heat transfer improvement, both laterally and transversely, over the stainless steel plate clad with carbon steel. The coefficient of thermal conductivity for copper is 226 Btu/sq. ft./ft./hr. .degree. F. vs. carbon steel of 27 Btu/sq. ft./ft./hr. .degree. F. Aluminum, although better than carbon steel, is not nearly as good as copper with a coefficient of thermal conductivity of 90 Btu/sq. ft./ft./hr. .degree. F. Aluminum also has the added disadvantage of its position in the galvanic series relative to stainless steel.
A comparison of the pertinent thermal properties of the foregoing metals is set forth in the following chart:
______________________________________ Density Coefficient of Thermal Specific Material lb./cu. in. Thermal Exp. Conductivity Heat ______________________________________ Copper CDA 110 0.323 9.8 226.0 0.092 Aluminum 6061-T6 0.098 13.5 90.0 0.230 Carbon Steel C1020 0.283 6.3 27.0 0.100 Stainless Steel 0.290 9.9 9.4 0.120 304/316 ______________________________________ Thermal Expansion per .degree. F. .times. 10(-6), temperature range 77.degree. F.-572.degree. F. Thermal Conductivity Btu/sq. ft./ft./hr. .degree. F. @ 68.degree. F.
As seen in the foregoing chart, the coefficients of thermal expansion for copper and stainless steel are much closer to each other than carbon steel is to stainless steel. Significant bowing of the bonded clad plate occurs as the temperature rises, with a 57% differential in the coefficients of thermal expansion between stainless steel and carbon steel. Aluminum, with a coefficient of thermal expansion that is 36% greater than that of stainless steel, will also cause bowing when aluminum is clad to stainless steel.
In the griddle of this invention, copper is used with stainless steel in order to take advantage of the heat transfer advantages of the copper, without any of the problems incumbent in cladding the stainless steel with copper. Instead, the copper is clamped against the stainless steel, without being bonded to the stainless steel. The copper layers are lubricated with a high temperature grease, so that they can move relative to each other and relative to the stainless steel. The net effect is significantly reduced bowing and potential warping of the stainless steel, as compared to clad plate.
One of the advantages of having the copper held against the stainless steel, but not bonded to the stainless steel, is that the griddle surface and sides can be formed from a sheet of stainless steel, with welds vertically in the four comers. This method eliminates welds to stainless steel sheet metal sides around the bottom of the clad plate. Failures of welds around the bottom of the clad plate occur due to thermal stresses at elevated temperatures, as described above. A significant advantage of the new design is the ability of the assembled pan to withstand sudden changes in temperature, e.g., cold water sprayed into a hot pan @400.degree. F., without the concern of welds failing.
Another advantage of utilizing copper for its thermal conductivity as opposed to aluminum and carbon steel, is that stainless steel and copper are much closer together in the galvanic series than either aluminum or carbon steel is relative to stainless steel. This becomes important if moisture is present, since corrosion can start at the interface of the assembly. This can result in galvanic corrosion.
The combination of the copper and stainless steel in this invention results in significantly greater heat distribution laterally, as compared with carbon steel. The heat distribution utilizing the copper is 3.3 to 6.3 times more effective than that which results from using the carbon steel. The composite copper thickness of 0.188" is equivalent to 1.569" carbon steel. Typical clad plate utilized in the industry employs between 0.250" and 0.500" carbon steel.
There is significantly increased conductivity (60%) through the composite copper and stainless steel, as compared to the stainless steel clad with carbon steel. Additionally, the composite copper/stainless steel results in faster recovery when heat is called for.
More accurate temperature readings are obtained with the thermobulb device attached to the copper. There is reduced bowing and warping, and weld failures are eliminated, by using the copper composite of this invention. Additionally, a smoother finish can be obtained for the stainless steel griddle surface, since the grinding and polishing of bottom welds is eliminated. This makes it easier to clean the griddle surface, and comply with federal, state and local food codes.
The draw-off for the grease on the griddle can be welded and blended into the bottom radius of the pan to allow easier, more complete draining when the pan is tilted. With a welded clad plate, the draw-off must be placed above the welds of the plate in order to provide strong welds. This placement can hinder complete draining.